1. Field of the Invention
The invention relates to a method for the continuous autogenous grinding of free-flowing stock containing insoluble particles of varying diameter and an apparatus for putting the method into practice.
2. Background Art
The grinding of especially hard substances such as silicates and carbides is complicated. Agitator mills are known to be used that comprise a cylindrical receptacle, in which a high-speed agitator unit is disposed concentrically. The grinding receptacle is at least substantially filled with auxiliary grinding bodies. The grinding stock is supplied to one end of the receptacle in a free-flowing condition - it may for instance be made into a paste with the addition of water - and it is discharged from the receptacle at the other end. The mix of grinding stock and auxiliary grinding bodies is intensively moved by the agitator unit so that intensive milling takes place. In the vicinity of the grinding-stock outlet, provision must be made for an auxiliary-grinding-body retaining device, by means of which the auxiliary grinding bodies can be separated from the stock for the latter to be discharged free from auxiliary grinding bodies. When extremely hard particles are milled, the wear of the auxiliary grinding bodies is high. As for the milling of lowgrade grinding stock as bulk goods, the cost of the wear of auxiliary grinding bodies is not reasonable as compared with the value of the grinding stock, even though the wear of auxiliary grinding bodies is not overly high. Moreover, there is quite a risk that the auxiliary-grinding-body retaining device is clogged by particles of grinding stock that are still too large and/or by worn auxiliary grinding bodies, which may lead to breakdowns or even to the partial destruction of the agitator mill. This risk increases when high throughputs of stock are run, which is accompanied by correspondingly high flow rates of the stock in the agitator mill. EP 0 219 740 B1 teaches an annular-gap-type ball mill for continuously pulverizing in particular hard mineral substances, comprising a closed grinding container housing a rotor, whose outer surface cooperates with the inner surface of the grinding container to define a grinding gap. The grinding gap contains so-called grinding pellets, i.e. auxiliary grinding bodies. The top portion and the lower portion of the rotor taper in opposite directions. As a result of the double-conical design of the grinding gap, any discharge of the auxiliary grinding bodies along with the stock through an outlet, and thus any reduction of the quantity of auxiliary grinding bodies or of the grinding effect is precluded. This is due to the fact that a given excess quantity of auxiliary grinding bodies collects in the radial annular chamber at the upper end of the grinding gap, i.e. where the diameter of the rotor has its maximum, there forming a floating blocking layer that will retain the active auxiliary grinding bodies in the grinding gap without affecting, in the way of a screen or the like, the discharge of the pulverized stock from the grinding gap in the direction of the outlet. There is no need of any subsequent separation of auxiliary grinding bodies and grinding stock. However, this is only true for low throughputs, i.e. at a low flow rate of the milled stock in the grinding gap. In the case of high throughputs and correspondingly higher flow rates of the stock in the grinding gap, the auxiliary grinding bodies will be discharged too, again necessitating the subsequent separation of auxiliary grinding bodies and stock. Of course, the above-mentioned problems of wear of the auxiliary grinding bodies continue to exist.